The invention relates to a process for preparing pure hydrofluoric acid or pure hydrogen fluoride from technical-grade hydrogen fluoride. In the production of hydrogen fluoride by reaction of fluorspar with sulphuric acid, the technical-grade hydrogen fluoride which is condensed contains, as secondary constituents, SO2, H2SO4, H2O, SiF4, PF3, POF3, PF5, BF3, AsF3 and other metal fluorides. Distillation enables both the low boilers SiF4, PF3, POF3, PF5, BF3 and SO2, and the high boilers H2SO4, H2O and other metal fluorides to be largely removed from the hydrogen fluoride. The main problem is the removal of the high boiler AsF3 which is difficult to bring down below 1 ppm (calculated as As) by distillation. The removal of As can be aided by oxidizing the trivalent As to pentavalent As prior to the high boiler distillation, e.g. by means of KMnO4, electrolytically or by means of fluorine (cf. AU 266 930, U.S. Pat. No. 5,108,559, U.S. Pat. No. 4,668,497). The hydrogen fluoride produced in this way is very pure but does not yet meet the high purity requirements of the electronics industry or analytical chemistry in respect of all parameters. The main problems are residual contents of SiF4 and of SO2 which can be oxidized to H2SO4 on mixing the HF with water. If the removal of high boilers is carried out in inexpensive steel apparatus instead of in very expensive, fluoropolymer-lined apparatus, increased heavy metal fluoride concentrations also have to be expected. Similarly, the storage of high-purity hydrogen fluoride before mixing with water requires expensive lined tanks.
As an alternative, the hydrogen fluoride can be mixed with water after the removal of the low boilers and the trivalent As in the aqueous hydrofluoric acid can be oxidized to pentavalent As, e.g. by means of KMnO4 (cf. GB 1 192 474; CS 240 051). The purification of this aqueous hydrofluoric acid by distillation likewise requires lining of the apparatus with expensive fluorinated polymers. However, the high working temperatures in the range from 90 to 110xc2x0 C. present a problem because the diffusion rate of HF through the lining materials increases sharply with rising temperature, so that damage to the apparatus can result. A further problem is the disposal of the metal fluoride-containing distillation residues which consist essentially of 40% strength HF and represent a relatively large proportion of the hydrofluoric acid used.
An object of the process of the invention is the reliable and economical preparation of pure hydrofluoric acid or pure hydrogen fluoride while avoiding the disadvantages of the known processes.
The present invention accordingly provides a process for preparing pure hydrofluoric acid or pure hydrogen fluoride from technical-grade hydrogen fluoride having an HF content of at least 97% by weight by
a) removal of low-boiling impurities by distillation,
b) subsequent oxidation of trivalent arsenic compounds to pentavalent arsenic compounds and, if desired,
c) removal of the high-boiling impurities by distillation,
characterized in that the hydrogen fluoride obtained either after step b) or after step c) is subjected to a two-stage scrub, in the first stage with 65-90% strength by weight hydrofluoric acid and in the second with 90-100% strength by weight hydrofluoric acid or hydrogen fluoride, with simultaneous cooling to from 10xc2x0 C. to 30xc2x0 C., and a small amount of water is fed into the first or second scrubbing circuit and a corresponding amount of 65-90% strength by weight hydrofluoric acid is bled off to remove impurities which are still present.